subpolar north atlantic

Phytoplankton diversity and contributions to carbon and nitrogen cycling

The subpolar North Atlantic is one of the best- and longest-studied regions of the ocean. Indeed, Sverdrup’s 1953 critical depth hypothesis, which examines the roles of nutrients and light in setting the timing of the spring phytoplankton bloom, was developed to explain observations in this region. Following a period of deep winter mixing that entrains subsurface nutrients into the mixed layer, the subpolar North Atlantic spring bloom, which is so large as to have biogeochemical consequences for the global ocean, develops progressively and relatively predictably from south to north between 45°N and 55°N. In direct contrast to the nutrient-poor Sargasso Sea, the phytoplankton bloom community in the subpolar North Atlantic is dominated by large eukaryotic phytoplankton. Diatoms have been reported to account for more than 50% of carbon fixation and nitrate assimilation during the springtime period of intense nutrient drawdown, while picocyanobacteria (i.e., Synechococcus) are responsible for only ~10% of surface chlorophyll and carbon fixation.

I am involved in a large, collaborative project led by Bess Ward that aims to investigate the taxonomic, genetic, and functional diversity of eukaryotic phytoplankton, and to link this diversity to carbon and nitrogen cycling in the surface ocean. This work is building on our research in the Sargasso Sea and extending it into the subpolar North Atlantic. My role is focused on linking genetic and taxonomic phytoplankton diversity to functional diversity through the application of natural abundance and tracer stable isotope methods in conjunction with flow cytometry.

figure 2: The δ15N of total bulk PN (black crosses), bulk PN >30 µm (grey triangles), and flow cytometrically sorted eukaryotic phytoplankton (green circles) and Synechococcus (red squares) at Stn 1 in September 2013. The δ15N of dissolved nitrate+nitrite (light purple profile) and nitrate-only (dark purple profile) is also shown; throughout the mixed layer, nitrate concentrations were ~2 µM. The approximate depth of the surface mixed layer (MLD) is indicated by the dashed grey line, and the fairly rapid attenuation of light in the upper water column is shown by the profile of photosynthetically active radiation (PAR; blue line). Figure from Fawcett et al., in prep.

We have undertaken two cruises in the subpolar North Atlantic as part of this project, the first in September 2013 (figure 1) and the second in May 2015. Guided by near-real time remote sensing imagery, samples were collected along a west-east transect from the coast of North America to the subarctic province and at two Process Stations (figure 2), where a number of experiments were also conducted. Samples were collected for: nutrient concentrations, pigments, phytoplankton community composition (via microscopy and flow cytometry), nitrate isotopes, particulate organic carbon and nitrogen content and isotopic composition, FACS, rates of carbon fixation and nitrogen assimilation, DNA and RNA, and dissolved inorganic carbon and alkalinity.

For the summertime transect, we find an inverse relationship between the abundance of Synechococcus and picoeukaryotic phytoplankton, with the eukaryotes becoming relatively more important to the east (figure 1B-C). The N isotopic composition (δ15N) of bulk particulate organic N (PN; figure 1B) and dissolved nitrate (NO3–; figure 1C) indicates a greater degree of nitrate utilization in subpolar waters, consistent with the higher nutrient supply to the surface (relative to the subtropics) that characterizes the high latitudes. The δ15N of flow cytometrically-sorted particles collected throughout the upper water column at Process Stn 1 (54°N; 20°W) suggests that Synechococcus and eukaryotic phytoplankton rely on different N sources to fuel their growth (figure 2). The δ15N of the eukaryotes (open green circles) resembles that of the nitrate supply to the base of the euphotic zone, whereas Synechococcus (open red squares) is much lower in δ15N, likely as a result of recycled N (mainly ammonium) assimilation. We expect the δ15N of ammonium to be lower than that of the bulk PN because of isotopic fractionation during the degradation and metabolism of PN by heterotrophic bacteria and zooplankton.